The present invention relates to a system for eliminating abnormal sounds in a record disc reproducing apparatus. More particularly it relates to a system for preventing the generation of unpleasant sounds at the start of reproducing a multichannel record disc on which an angle-modulated wave has been recorded, or at the time of searching for detection of the starting point of a sound recording.
In general, there are instances when a pickup of a disc reproducing apparatus is left on a soundless groove in front of the point where a sound recording begins on a recorded disc. With the pickup at this point, the rotation of the record disc is started to reproduce or play the sound recording. Such a procedure is also carried out with a two-channel record disc reproducing apparatus, of ordinary type for home use. Furthermore, this kind of start is also used during a disc jockey program in a broadcasting station.
On a discrete multichannel record, an angle-modulated wave signal is recorded in a superimposed state with a direct wave signal. An unmodulated carrier component of 30 KHz is recorded in the soundless part of the groove. A discrete multichannel record can be reproduced also in an apparatus for reproducing two-channel stereo record discs.
While the pickup is tracing the soundless groove part of the record disc, at the start of reproduction by the above mentioned procedure, the disc starts rotating from its stopped state, comes up to speed, and eventually reaches its normal rotational speed. Accordingly, until the disc reaches this normal speed, it is rotating at lower than normal speeds. While the disc is thus rotating at lower speeds, the carrier component is reproduced as a signal of a frequency in the audio frequency band and is heard as an abnormal and unpleasant sound (sweep sound).
The above mentioned search for the starting point of the sound recording is accomplished by turning the turntable, on which the record disc is placed, in either of the two opposite directions. During this procedure, abnormal sound is also generated to give an unpleasant sensation and difficulty in finding the starting point for the sound recording.
In order to eliminate such abnormal sounds, a known system detects the difference between the waveform of the carrier wave in the sound containing groove part and the waveform of the carrier wave in the soundless groove part. The reproduced signal transmission path is cut off while the pickup is tracing the soundless part of the groove. This prevents the reproduction and generation of abnormal sounds. More specifically, only unmodulated carrier waves of the same phase are recorded on the left and right channel walls in the soundless part of the groove. A carrier wave angle modulated by a difference signal and multiplexed on a direct wave sum signal is recorded in the sound containing groove part. For this reason, the difference between the left and right channel signals is determined. The difference becomes zero responsive to the soundless groove part. There is a difference output responsive to the sound containing groove part. Therefore, the soundless groove part and the sound containing groove part are distinguished and detected.
In the above described known system, the carrier waves recorded in the left and right channels in the soundless groove part of the disc are not necessarily of mutually the same phase. Even if they are of the same phase, the carrier waves of the left and right channels, reproduced by the pickup, may become different in phase as a result of the characteristic of the pickup. Consequently, the distinguishing and detection of the soundless groove part and the sound containing part cannot be carried out positively. The transmission path of the reproduced signal cannot be positively cut off while the pickup is tracing the soundless groove part.
In this connection, the generation of the above mentioned abnormal sounds may be considered as follows. The frequency of the unmodulated carrier wave recorded on a discrete four-channel record, for example, is 30 KHz. For this reason, if the record disc is rotated at less than the normal speed, the carrier wave component is reproduced as a signal within the frequency range of 0 to 30 KHz. If the carrier wave thus reproduced is in the audio frequency band, particularly within the frequency band of from 300 Hz to 10 KHz, the sound of the reproduced carrier wave is sensed as an abnormal sound of maximum unpleasantness.
The greater part of the energy of the audio signal resides in a band from 500 Hz to 5 KHz. If the carrier wave is reproduced with a 300 Hz to 10 KHz frequency, the greater part of the energy of the audio signal shifts to approximately 50 Hz to 1.7 KHz, and is thus reproduced.
Accordingly, one measure for eliminating abnormal sounds due to reproduction of the carrier wave at a frequency in the audio band when the record disc is rotated below the normal speed, is to increase the signal passing band of the reproducing signal transmission system from approximately 100 Hz, as the reproduced carrier wave frequency increases.
As a result of experiments relating to aural sensation, it was found that, while audio signals can be distinguished, the sound of the reproduced carrier wave can be prevented from being heard by attenuating the level of the reproduced carrier wave to less than 40 dB.